In many investigations, by way of computed tomography, the patient is administered a contrast agent in order, in particular, to be able to identify blood vessels clearly in the CT images obtained. However, when viewing the CT images other body constituents than bone can have a disturbing effect, since they likewise strongly absorb the X-radiation. Such areas are therefore removed from the images during image reprocessing, if appropriate by computer devices. Again, the identification of other calcium-containing areas, or ones enriched with calcium, such as plaques in vessels or calcium-containing kidney stones requires a distinction to be made between contrast agent and these calcium-containing areas or materials, in order to be able to highlight these, for example when displaying them in a CT image. However, a semiautomatic or automatic segmentation of bones or other calcium-containing areas requires contrast agent and the calcium-containing material in the respective image data to be distinguished as automatically as possible.
Various approaches have been adopted to date in order to distinguish these materials. Thus, a CT scan can be carried out in each case before and after the administration of the contrast agent. The raising of the X-ray attenuation value by the contrast agent is measured by image registration and subtraction of the two CT images. Bones or other calcium-containing areas appear in this case as unchanged regions with a high X-ray attenuation value. A further possibility resides in carrying out a classification in the CT image of a single CT scan on the basis of the level of the X-ray attenuation values. However, this frequently requires additional evaluation of local statistical variables or morphological features in order to obtain results that are to some extent reliable.